Methods respectively for manufacturing hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet

ABSTRACT

A method for manufacturing a hot-dip galvanized steel sheet according to one aspect of the present invention includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.

TECHNICAL FIELD

The present invention relates to methods for manufacturing a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet.

BACKGROUND ART

In recent years, high-tensile strength steel sheets have been used for automobile members for the purpose of improving fuel efficiency by weight reduction and collision safety performance. In order to manufacture a high-tensile strength steel sheet, various reinforcing elements are often contained in steel.

As such reinforcing elements, especially, Mn and Si are known as inexpensive and effective elements. Among these, however, Si is oxidized outward to a surface of a steel sheet during reduction annealing, so that it forms an oxide film to inhibit wettability with molten zinc during plating and cause an appearance failure such as bare spots and uneven alloying. Therefore, in Si-containing steel, it is difficult to produce a hot-dip galvannealed steel sheet having a good appearance.

As a method for manufacturing a hot-dip galvanized steel sheet or a hot-dip galvannealed steel sheet of Si-containing steel having a beautiful appearance, the following methods have been proposed.

For example, Patent Literature 1 proposes a method that involves controlling a temperature of a steel sheet entering into a plating bath. Specifically, there is proposed a method for manufacturing a hot-dip galvanized steel sheet in which a steel sheet having a Si content of 0.05% by weight or more is plated by being immersed in a hot-dip galvanizing bath having a bath temperature of 440° C. or higher and containing 0.05 to 0.5% of Al at an entering sheet temperature T satisfying 350+30/t≤T(° C.)≤420+30/t, wherein t is a sheet thickness (mm), and T(° C.)≤460. However, this condition can suppress the occurrence of bare spots, but may make the plating thickness uneven and fail to obtain a beautiful appearance.

Patent Literature 2 proposes a method for manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance by using a base steel sheet having a Si content of 0.8 to 2.5% by mass and by controlling, in a redox plating process, a Fe-based oxide film thickness, a plating bath temperature, a temperature of a steel sheet entering into a plating bath, and an effective Al concentration in the bath. In a redox plating process, however, a step of oxidizing a steel sheet is required, and it is difficult to stably and appropriately control an oxide film thickness and to appropriately control the reduction amount according to the oxide film thickness, and it is difficult to stably manufacture a hot-dip galvannealed steel sheet having a beautiful appearance.

Patent Literature 3 proposes a method for manufacturing a hot-dip galvanized steel sheet with no bare spots by plating a steel sheet having a Si content of 0.5 to 2.0% under very high entering sheet temperature conditions satisfying T(Zn)+100° C.≤T≤T(Zn)+180° C. and 440° C.≤T(Zn)≤470° C. wherein T is a temperature (° C.) of a steel sheet entering into a plating bath and T (Zn) is a bath temperature (° C.) of the plating bath. The occurrence of bare spots can be suppressed by this method, but it is difficult to appropriately control a plating deposition weight by wiping after plating due to the progress of alloying in the plating bath, and uneven alloying may occur.

In addition to the above methods, a high dew point plating method is known as one of the methods of plating on Si-containing steel. The high dew point plating method is a method in which Si in steel is internally oxidized by increasing an atmosphere dew point in an annealing furnace, and plating is performed while suppressing outward oxidation. When using the high dew point plating method, plating can be applied even on high Si steel having a Si content of 1.0% by mass or more. However, when the annealing temperature of a steel sheet is A3 point or higher in the case of using the high dew point plating method, the resulting hot-dip galvannealed steel sheet has an appearance in which ash floating in a zinc hot-dip galvanizing bath is attached, and there is a problem that the appearance is significantly deteriorated.

In addition, Patent Literature 4 proposes a method for manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance by using a steel sheet having a relatively small Si content of 0.1% by mass or less and controlling the temperature of the steel sheet entering into a plating bath. In this method, since the content of Si in the steel sheet to be used is small and the amount of outward oxidation formed during annealing is small, plating can be performed without using the high dew point plating method, and an appearance failure does not occur in the resulting plated steel sheet. However, the plated steel sheet obtained using the method described in Patent Literature 4 has a problem that sufficient strength cannot be obtained because the content of Si in the steel is small.

As described above, when a steel sheet of high Si steel having a Si content of 1.0% by mass or more is used, there is a problem that it is difficult to stably manufacture a hot-dip galvannealed steel sheet having a beautiful appearance.

The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a method for manufacturing a hot-dip galvanized steel sheet capable of stably manufacturing a hot-dip galvanized steel sheet that can be used as a material of a hot-dip galvannealed steel sheet having a beautiful appearance using high Si steel, and a method for manufacturing a hot-dip galvannealed steel sheet capable of stably manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-2000-303158

Patent Literature 2: JP-A-2010-189734

Patent Literature 3: JP-A-2010-156029

Patent Literature 4: JP-A-H7-216526

SUMMARY OF INVENTION

As a result of various studies, the present inventors have found that the above object is achieved by the following invention.

A method for manufacturing a hot-dip galvanized steel sheet according to one aspect of the present invention includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.

A method for manufacturing a hot-dip galvannealed steel sheet according to another aspect of the present invention includes an alloying step of alloying the galvanized layer formed in the hot-dip galvanized steel sheet obtained by the aforementioned method for manufacturing a hot-dip galvanized steel sheet.

The foregoing and other objects, features, and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are optical micrographs of a cross section of a steel sheet after annealing, where FIG. 1A shows a case where an annealing temperature is A3 point or higher, and FIG. 1B shows a case where the annealing temperature is lower than A3 point.

DESCRIPTION OF EMBODIMENTS

The present inventors have repeatedly studied a method for manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance using a steel sheet of high Si steel having a Si content of 1.0% by mass or more as follows.

As a method for stably forming a plated layer having a beautiful appearance on a steel sheet of high Si steel, a high dew point plating method that involves increasing the atmosphere dew point during annealing is known. In the high dew point plating method, since the entire steel sheet is exposed to a uniform atmosphere in an annealing furnace, the appearance of plating and the thickness of a plated layer become uniform.

However, even in the high dew point plating method, when the annealing temperature is equal to or higher than the A3 point of the steel, the appearance failure of the resulting hot-dip galvannealed steel sheet may occur.

First, as a result of an investigation of the appearance of the hot-dip galvannealed steel sheet, it was found that, in a snout for guiding the steel sheet to a galvanizing bath, ashes floating on the galvanizing bath adhered to a plated layer, so that an appearance failure occurred.

Next, in order to confirm the influence of the annealing temperature, the steel sheet of the high Si steel was annealed with the dew point of the annealing atmosphere set to 0° C. and with the annealing temperature set to lower than the A3 point or set to the A3 point or higher. As a result of observing a cross section of the annealed steel sheet with an optical microscope, when the annealing temperature was the A3 point or higher, the internal oxide layer generated at the surface of the steel sheet was in a state dominated by grain boundary oxidation as shown in FIG. 1A. On the other hand, when the annealing temperature was lower than the A3 point, the internal oxide layer generated at the surface of the steel sheet was in a state dominated by in-grain oxidation as shown in FIG. 1B. That is, it was found that when the annealing temperature was the A3 point or higher, the amount of oxide generated on the surface of the steel sheet increased because grain boundary oxidation dominated.

A mechanism in which an appearance failure occurs in a plated steel sheet obtained by a high dew point plating method at the time of high temperature annealing at the A3 point or higher is not clear. However, the form of an internal oxide layer of a steel sheet was dominated by in-grain oxidation in annealing at the A3 point or lower, whereas it was dominated by grain boundary oxidation in high temperature annealing. From this, it is presumed that this is because the reactivity between the surface of a steel sheet and ashes in a snout was improved due to the influence of the form of internal oxidation and the amount of oxide on the surface of the steel sheet was increased by high temperature annealing, so that the ashes were facilitated to adhere to the surface of the steel sheet.

Further intensive studies based on these investigations and the results of studies have revealed that, in the case of setting the annealing temperature to the A3 point or higher and performing plating by a high dew point plating method, a hot-dip galvannealed steel sheet having a beautiful appearance can be stably obtained even when high Si steel is used due to setting the temperature of the steel sheet entering into a plating bath to 390° C. or lower.

The present inventors have accomplished the present invention based on these findings.

According to the present invention, it is possible to stably manufacture a hot-dip galvanized steel sheet which can be used as a material of a hot-dip galvannealed steel sheet having a beautiful appearance by using high Si steel.

In addition, according to the present invention, it is possible to stably manufacture a hot-dip galvannealed steel sheet having a beautiful appearance using high Si steel.

Hereinafter, embodiments of the present invention will be specifically described, but the present invention is not limited thereto.

(Method for Manufacturing Hot-Dip Galvanized Steel Sheet)

A method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower. Thus, a hot-dip galvanized steel sheet that can be used as a material of a hot-dip galvannealed steel sheet is stably obtained.

Hereinafter, the reason why the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment has been defined as described above will be described.

(Annealing Step)

In the annealing step, a steel sheet of high Si steel having a Si content of 1.0% by mass or more and 3.0% by mass or less is subjected to reduction annealing at a temperature equal to or higher than the A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher. For the reduction annealing, for example, an indirect heating type annealing furnace such as an all radiant type annealing furnace can be used. The steel sheet of the high Si steel to be used may be a steel sheet manufactured by a conventional method and, and can be obtained, for example, by smelting a cast piece of steel satisfying the Si content described above, and then subjecting the cast piece to hot-rolling, pickling, and cold-rolling.

In the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment, it is important to perform annealing at a temperature appropriate for manufacturing a high-strength steel sheet. In the present embodiment, the annealing temperature is set to an A3 point or higher in order to obtain a steel sheet having desired strength characteristics. In this case, the upper limit of the annealing temperature is not particularly determined as long as the annealing temperature is a temperature at which no liquid phase is generated. However, it is desirable that the annealing temperature is lower than 1000° C. in consideration of the durability of the annealing furnace. The holding time at the annealing temperature is preferably 1 s or more and preferably 600 s or less from the viewpoint of obtaining a steel sheet having desired strength characteristics.

The A3 point can be calculated by the following formula (i) (“The Physical Metallurgy of Steels” (published by Maruzen Co., Ltd., written by William C. Leslie, p. 273)). The element symbol enclosed in [] in the formula (i) represents the content (% by mass) of the element.

A3 (° C.)=910−203×[C]^(1/2)−15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]−{30×[Mn]+11×[Cr]+20×[Cu]−700×[P]−400×[Al]−120×[As]−400×[Ti]}  (i)

The dew point of the annealing atmosphere is set to −20° C. or higher. Thus, a hot-dip galvanized steel sheet as a material of a hot-dip galvannealed steel sheet having a beautiful appearance can be stably manufactured in the subsequent hot-dip galvanizing step. This is because when the dew point of the annealing atmosphere is high, the oxygen potential in the annealing atmosphere increases, and Si and oxygen solid-dissolved in the steel sheet react with each other to form internal oxidation, so that solid-dissolved Si that adversely affects the plating property can be rendered harmless. In this case, the entire steel sheet is exposed to a uniform atmosphere in the annealing furnace and falls into a uniform state, so that the appearance of the plating formed in the hot-dip galvanizing step and the thickness of the plated layer are uniform. In order to suppress oxidation of the steel sheet, the upper limit of the dew point of the annealing atmosphere is preferably 10° C. The annealing atmosphere can be, for example, a reducing atmosphere of N₂-5 vol % H₂.

(Hot-Dip Galvanizing Step)

In the hot-dip galvanizing step, the steel sheet is made to enter a galvanizing bath, and a galvanized layer is formed on the surface of the steel sheet to complete a hot-dip galvanized steel sheet. As the galvanizing bath, one used for manufacturing a normal hot-dip galvanized steel sheet can be used.

In the present embodiment, the temperature of the steel sheet entering into the galvanizing bath (hereinafter, the temperature is also referred to as “entering sheet temperature”) is 390° C. or lower. Thus, the appearance failure of a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet obtained using the same as a material is suppressed. The reason why the appearance failure of these plated steel sheets is suppressed by setting the entering sheet temperature to 390° C. or lower is considered to be that the reactivity between the surface of the steel sheet and ashes in a snout decreases even when the mode of the internal oxidation of the steel sheet generated during the annealing step is mainly grain boundary oxidation. However, when the entering sheet temperature is excessively low, the energy consumption rate required to maintain the temperature of the galvanizing bath cooled by the steel sheet increases. Therefore, the entering sheet temperature is preferably 350° C. or higher. The immersion time of the steel sheet in the galvanizing bath can be adjusted according to a desired plating deposition amount.

(Method for Manufacturing Hot-Dip Galvannealed Steel Sheet)

The method for manufacturing a hot-dip galvannealed steel sheet according to the present embodiment includes an alloying step of alloying the galvanized layer formed on the hot-dip galvanized steel sheet obtained by the method described above. That is, the method for manufacturing a hot-dip galvannealed steel sheet according to the present embodiment includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher; a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet; and an alloying step of alloying the galvanized layer, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower. Thus, a hot-dip galvannealed steel sheet having a beautiful appearance can be stably obtained.

(Alloying Step)

In the alloying step, the hot-dip galvanized steel sheet is heated to a prescribed alloying temperature to diffuse iron atoms constituting the steel sheet into the plated layer, thereby alloying the plated layer. The alloying temperature is preferably 400° C. or higher and preferably 600° C. or lower. The time for holding at the alloying temperature (hereinafter, it is also referred to as “alloying time”) is preferably 1 s or more and preferably 60 s or less from the viewpoint of optimizing the alloying state. The heating atmosphere may be atmospheric air.

In the method for manufacturing a hot-dip galvannealed steel sheet according to the present embodiment, the hot-dip galvanizing step and the alloying step are preferably performed using a continuous galvanizing line (CGL). Thus, the hot-dip galvanizing step and the alloying step can be continuously performed in a continuous manufacturing line, so that the productivity of the hot-dip galvannealed steel sheet can be improved.

(Chemical Composition of Steel Sheet)

Si: 1.0% by mass to 3.0% by mass

In the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment, the Si content in the steel sheet before the formation of the galvanized layer is 1.0% by mass or more and 3.0% by mass or less. Si is an element that has high solid solution reinforcing performance in a steel sheet and enhances strength without reducing the ductility of the steel sheet. From the viewpoint of securing the strength of the steel sheet, the lower limit of the Si content of the steel sheet is set to 1.0% by mass. However, if the Si content is excessive, the strength of the steel sheet is excessively high, so that the rolling load increases, and Si scales are generated during hot-rolling to deteriorate the surface quality of the steel sheet. In addition, the wettability of molten zinc decreases during hot-dip galvanization, and it becomes very difficult to perform hot-dip galvanization. Therefore, the upper limit of the Si content is set to 3.0% by mass. The lower limit of the Si content is preferably 1.1% by mass and more preferably 1.2% by mass. The upper limit of the Si content is preferably 2.7% by mass and more preferably 2.5% by mass.

The balance of the steel sheet other than Si contains Fe as a main component and also contains unavoidable impurities.

In the steel sheet, the contents of C and Mn may be set as follows from the viewpoint of securing excellent mechanical properties.

C: 0.05% by mass to 0.5% by mass

C is an element that enhances the strength of the steel sheet. When the steel sheet is used as a high-strength steel sheet, the C content is preferably 0.05% by mass or more. On the other hand, when the C content is excessive, weldability deteriorates. Therefore, the C content is preferably 0.5% by mass or less.

Mn: 1.6% by mass to 4.0% by mass

Mn is an element that enhances the strength of the steel sheet and promotes the generation of retained austenite to enhance the workability. When the steel sheet is used as a high-strength steel sheet, the Mn content is preferably 1.6% by mass or more. On the other hand, when the Mn content is excessive, ductility and weldability deteriorate. Therefore, the Mn content is preferably 4.0% by mass or less.

The steel sheet may further contain, for example, one or more of the following elements.

Al: 0.001% by mass to 0.5% by mass

Al is an element that acts as a deoxidizing agent in smelting steel. In the case of deoxidizing with Al, the Al content is preferably 0,001% by mass or more in order to effectively exhibit the effect. On the other hand, when the Al content is excessive, much inclusions such as alumina are generated in the steel sheet, and the workability may be deteriorated. Therefore, the Al content is preferably 0.5% by mass or less.

Cr: 0.01%© by mass to 0.3% by mass

Cr is an element that suppresses oxidation. When the steel sheet contains Cr, the amount of Si oxide generated at grain boundaries is reduced, so that the amount of solid solute Si increases. Both of the solid solute Si and Cr act as an oxidation inhibiting element to prevent rapid progress of oxidation in an oxidation step. In order to exhibit such an effect, the Cr content is preferably 0.01% by mass or more. On the other hand, when the steel sheet excessively contains Cr, the progress of oxidation is greatly suppressed to cause insufficient oxidation. Therefore, the Cr content is preferably 0.3% by mass or less.

Ti: 0.0005% by mass to 0.2% by mass

Ti is an element that forms a carbide or a nitride to improve the strength of a steel sheet. It is also an element for forming Ti nitride to reduce the N content in steel, thereby suppressing the formation of B nitride and effectively utilizing the quenching property of the solid solute B. In order to effectively exhibit such an effect, the Ti content is preferably 0.0005% by mass or more. On the other hand, if the Ti content is excessive, the Ti carbide and the Ti nitride will be excessive, and the ductility, the stretch flangeability, and the stretch workability will be deteriorated. Therefore, the Ti content is preferably 0.2% by mass or less.

The unavoidable impurities refer to elements brought into steel depending on the situation of raw materials, materials, manufacturing facilities, etc., and examples thereof include S, N, O, Pb, Bi, Sb, and Sn in addition to P described below.

P: 0.03% by mass or less

P is an clement which a steel sheet contains as an unavoidable impurity. P not only segregates at grain boundaries to promote grain boundary embrittlement, but also deteriorates hole expandability. Therefore, the P content is preferably as low as possible, for example, 0.03% by mass or less.

The present description discloses the techniques of various aspects as described above, and the main techniques among them are summarized below.

As described above, the method for manufacturing a hot-dip galvanized steel sheet according to one aspect of the present invention includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.

With such a configuration, a hot-dip galvanized steel sheet as a material of a hot-dip galvannealed steel sheet having a beautiful appearance can be stably manufactured.

A method for manufacturing a hot-dip galvannealed steel sheet according to another aspect of the present invention includes an alloying step of alloying the galvanized layer formed in the hot-dip galvanized steel sheet obtained by the above-described method for manufacturing a hot-dip galvanized steel sheet.

With such a configuration, a hot-dip galvannealed steel sheet having a beautiful appearance can be stably manufactured.

In the above configuration, in the method for manufacturing the hot-dip galvannealed steel sheet, the hot-dip galvanizing step and the alloying step may be performed using a continuous galvanizing line.

Thus, the formation of hot-dip galvanizing on a steel sheet and the alloying of the formed galvanized layer can be continuously performed, and a hot-dip galvannealed steel sheet having a beautiful appearance can be efficiently obtained.

Hereinafter, the present invention will be more specifically described by way of examples; however, the present invention is not limited by the following examples, and can be carried out with changes within a scope that meets the gist described above and below, and such changes are all included within the technical scope of the present invention.

Examples

(Test Conditions)

(Composition of Steel)

Cast pieces manufactured so as to have the compositions of steel types A and B shown in Table 1 were smelted, and the cast pieces smelted were subjected to hot-rolling, pickling, and cold-rolling, to thereby obtain steel sheets of high Si steels. Table 1 also shows the A3 points of the respective steel types calculated by the formula (i).

TABLE 1 Composition (% by mass), balance: Fe and unavoidable impurities A3 point Steel type C Si Mn P Al Cr Ti (° C.) A 0.209 1.85 2.10 0.05 0.04 — — 858 B 0.095 1.80 2.10 0.01 0.045 0.2 0.06 890

The obtained steel sheet was sequentially subjected to reduction annealing and hot-dip galvanization using a continuous galvanizing line (CGL) equipped with an all radiant tube type annealing furnace, to obtain a hot-dip galvanized steel sheet. Furthermore, alloying treatment was continuously performed on the resulting hot-dip galvanized steel sheet, to obtain a hot-dip galvannealed steel sheet.

(Reduction Annealing Conditions and Hot-Dip Galvanizing Conditions)

Table 2 shows the steel types and the A3 points of the steel sheets, the dew points of the annealing atmospheres, the annealing temperatures, and the temperatures of the steel sheets entering into the galvanizing bath used in Test Nos. 1 to 6.

TABLE 2 Steel A3 point Dew point Annealing Entering sheet Test No. type (° C.) (° C.) temperature (° C.) temperature (° C.) Appearance 1 A 858 8 880 350 Good 2 A 858 6 880 380 Good 3 A 858 6 880 390 Good 4 A 858 5 880 400 Poor 5 A 858 7 880 460 Poor 6 B 890 −1 895 460 Poor

Conditions other than the conditions shown in Table 2 were common to all test Nos. and were as follows.

(Reduction Annealing Conditions)

Annealing atmosphere: N₂-5 vol % H₂

Annealing temperature: A3 point or higher

Holding time: 200 s

(Hot-Dip Galvanizing Conditions)

Al content in galvanizing bath: 0.13% by mass (balance being Zn and unavoidable impurities)

Galvanizing bath temperature: 460° C.

Immersion time in galvanizing bath: 4 s

(Alloying Conditions)

Atmosphere: atmospheric air

Alloying temperature: 460° C.

Alloying time: 27 s

(Evaluation Conditions and Test Results)

The hot-dip galvannealed steel sheets of the respective test Nos. obtained under the conditions described above were visually observed, and the appearance thereof was evaluated. The evaluation results are shown in Table 2. In Table 2, cases with a beautiful appearance are indicated by “Good”, and cases with an appearance failure are indicated by “Poor”.

As shown in Table 2, in Test Nos. 1 to 3 satisfying the conditions in the present invention, the appearance failure of the hot-dip galvannealed steel sheets was suppressed, and the steel sheets were good in appearance.

On the other hand, in Test Nos. 4 to 6 in which the temperature of the steel sheets entering into the galvanizing bath was higher than 390° C., the appearance was poor.

This application is based on Japanese Patent Application No. 2019-128698 filed on Jul. 10, 2019, the contents of which are included in the present application.

To describe the present invention, the invention has been described in the foregoing description appropriately and sufficiently using embodiments with reference to specific examples and the like. However, it is to be understood that changes and/or modifications to the foregoing embodiments will readily occur to those skilled in the art. Therefore, unless a change or modification made by those skilled in the art is beyond the scope of the appended claims, such change or modification is to be embraced within the scope of the appended claims.

INDUSTRIAL APPLICABILITY

The present invention has broad industrial applicability in the technical field related to methods for manufacturing a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet. 

1. A method for manufacturing a hot-dip galvanized steel sheet, the method comprising: an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.
 2. A method for manufacturing a hot-dip galvannealed steel sheet, the method comprising an alloying step of alloying the galvanized layer formed in the hot-dip galvanized steel sheet obtained by the method for manufacturing a hot-dip galvanized steel sheet according to claim
 1. 3. The method for manufacturing a hot-dip galvannealed steel sheet according to claim 2, wherein the hot-dip galvanizing step and the alloying step are performed using a continuous galvanizing line. 